First is a definition for the window itself, or rather for a handle used to point to the window’s definition:

DEFINE VARIABLE CustWin AS WIDGET-HANDLE NO-UNDO.

The variable CustWin is defined as a WIDGET-HANDLE. The term widget applies to all sorts of things, including visual objects such as fields and buttons, as well as other things that can have handles, including queries and even procedures themselves. WIDGET-HANDLE is a synonym for the keyword HANDLE, because a single ABL data type accommodates all these different kinds of objects. Since there is really just one HANDLE data type, and since it is used for more than just objects that you might consider widgets, this guide always uses the keyword HANDLE.

In Create the window, you look a little more about what the handle does when you get to the code that uses it to create the window.